International application WO 99/29772 describes the impact strengthening of semicrystalline thermoplastic resins using block copolymers of SBM type (styrene-butadiene-methyl methacrylate block copolymers).
International application WO 02/055573 of the Applicant Company describes the impact strengthening of a methacrylate home- or copolymer using a block copolymer of ABA type in which B denotes a block obtained from a diene, for example an SBM.
International application WO 03/062293 of the Applicant Company describes a process of the impact strengthening of a thermoplastic matrix using a B(-A)n block copolymer composed of a block B and of n branches A and prepared using the controlled radical polymerization technique. This process applies to the strengthening of numerous thermoplastics (PS, PC, PVDF, and the like) and in particular to the manufacture of cast PMMA sheets.
The process of WO 03/062293 applied to the manufacture of cast sheets is not capable of being transferred to the industrial scale. This is because it exhibits the disadvantage of requiring a stage of removal of solvent, followed by a stage of redissolution of the copolymer. First, these two unit operations, by increasing the overall cycle time, affect the yield of the process. Secondly, the stage of removal of solvent is also capable of resulting in the formation of gels in the B(-A)n copolymer, which affects its redissolution in the mixture of monomers and, consequently, can damage the transparency of the cast sheet.
Furthermore, according to the process described, in particular in the examples, it is preferable, during the 2nd stage, to initiate the formation of the branches A at the same time as that of the matrix. For this, the monomer A is brought into contact with two types of initiators, the conventional radical initiator and the reactivatable block B. The monomer A is thus consumed at the same time according to two competing radical polymerization mechanisms, each exhibiting its own kinetics. The control of this 2nd stage is highly problematic as it implies matching the rates of formation of the blocks A and the matrix. This implies that it is necessary to adjust the nature of the radical initiator to the block B and thus also to carefully adjust the temperature cycle. In practice, contradictory requirements are encountered and the possible compromises generally result:                in a premature separation during polymerization of the copolymer B(-A)n, which migrates to the interface of the sheet and mold. In this case, sheets are obtained which are impossible to remove from the mold and/or which are partially or completely opaque;        in unacceptable contents of residual methyl methacrylate, which it is impossible to remove once the sheet is complete.        
An improvement is introduced in patent EP 1 858 939. The cycle time of the process of this patent is improved with respect to that described in WO 03/062293 as it does not require any stage of removal of solvent/redissolution.
However, the mechanical properties of the materials obtained with the processes described above are not entirely satisfactory and an improvement in the properties, such as impact strength and the flexural modulus, is desired.
The Applicant Company has now discovered that major improvements in the impact strength and in the flexural modulus are observed when some compositions comprise crosslinking agents.
While the abovementioned documents often mention the possible use of crosslinking agent, none gives examples of or describes the properties of compositions comprising them, in particular the advantage presented by the use of crosslinking agent with a view to a combined increase in the flexural modulus and the impact strength.